I usually play with two AI players (more than that and the waiting is too long for my liking) and so far I win seldomly, but I'm not a hardcore tactician or anything remotely like that. I'm happy with the AI. When I win, I'm usually only a few points ahead.

I think I've played 5 games so far and won 3 of them. I think the games I lost were on easy and medium AI and I've been winning all the ones on hard :/. I have mostly been playing against 4 AI and I think it may be the most challenging with 1 or 2 since some cards give benefits to all players. However, it isn't really a particularly competitive game and the AI is decent so I don't personally mind that it could be a bit better.

Edit: And just played another vs 4 hard AI and tied for 4th, so maybe it was just luck that I was doing so well before :).

Edit2: Also, there is Automa, which (I had to look up since it isn't explained in this version of the game at all) is the single player mode from the board game and has its own rules designed to be quick to play, have some effects on shared resources, and end up with a point range that is a resonable challenge (also, regular bird powers only apply to you, not Automa). There is easy/medium/hard that determines the number of points its score increases on a default turn where it isn't doing anything else. Hard Automa does seem to be fairly hard so that is another available challenge. It is much faster than playing the regular AI and that remids me that IMO a faster version of the current difficulty AI would make the game more fun than a harder difficulty AI, at least for me.

Edit3: Last one, I promise :). Another game vs 4 hard AI and came in last. I'm not that great at these but I have played a number of strategy games so unless I'm having a particularly bad day I think the AI does give a decent challenge.